1. Field of the Invention
This invention relates to an information recording medium such as an optical disk and an optical recording and readout system which optically records information onto such an information recording medium and optically reads out the recorded information.
2. Description of the Related Art
Recently, tremendous research and development efforts have been directed toward high density recording and readout techniques in the field of information recording and readout equipment using optical information recording mediums. For signal recording techniques, the mark position recording method that causes a signal to correspond to the center of a recording mark has been largely replaced with the mark edge recording method that causes a signal to correspond to a recording mark edge to achieve much higher recording density. Also, efforts are now being put into the development of much larger capacity optical disks by recording signals between tracks.
As a first conventional example, a method of achieving high track density using the sample servo system has been described in Moore, G. S., "High Density Format for Sperry Information Storage Inc. Second Generation Optical Drives," SPIE Vol. 695, pp230-238, 1986. With this method, recording marks are formed on a recording medium so as to be 180 degrees out-of-phase track by track. When the readout optical beam is positioned at the center of a recording mark significant for an information readout signal, the readout optical beam does not overlap with the recording marks located in adjacent tracks, thereby in reducing crosstalk. In this way, crosstalk from both adjacent tracks is decreased by changing the recording clock timing between even-numbered tracks and odd-numbered tracks, thereby achieving a large capacity as compared with a conventional equivalent.
A second conventional example has been described in K. Kayanuma et al., "High Track Density Magneto-Optical Recording using a Crosstalk Canceler," SPIE Vol. 1316, pp35-29, 1990. According to this example, three optical beams are caused to follow an inner-circumference land track, a central groove track, and an outer-circumference land track respectively, these tracks adjoin each other. The readout signals obtained by the three optical beams are adjusted in phase. Then, the two phase-adjusted readout signals obtained by the optical beams on both sides are adjusted in gain. The gain-adjusted signals are added to each other. The added signal is subtracted from the readout signal obtained by the central optical beam, thereby producing a readout signal by means of the central optical beam with reduced crosstalk.
In the case of applying the first conventional example to the mark edge recording method, when the readout optical beam is located at the edge portion of a prepit significant for an information readout signal, simply shifting the phase of the prepit 180 degrees permits the readout optical beam to illuminate two prepits in adjacent tracks, causing noticeable crosstalk. This not only reduces the reliability of the information readout signal, but also makes it difficult to apply this example to the mark edge recording method capable of realizing high density.
For the land & groove recording method explained in the second conventional example, it is necessary to eliminate crosstalk from adjacent tracks on both sides in the complicated three-beam optical head and the signal processing circuit. When the second conventional example is applied to an optical disk where the presence/absence of prepits or recording marks is sensed by the change of reflectivity, such as a readonly optical disk, a write once optical disk, or a phase-change optical disk, crosstalk caused by prepits or recording marks located on the adjacent tracks on both sides reduces the amplitude of the focus and track error signals seriously, which makes the focus and track servos unstable, thus reducing the reliability of the information readout signal.